1. Field of the Invention
The present invention relates to a spool-type flow adjusting valve, preferably to be used in a die casting machine, which valve works effectively to adjust the flow rate in a hydraulic circuit and to switch an injection operation from low speed to high speed.
2. Description of the Related Art
In the die casting machine, melt is generally injected into the cavity of a metal mold from an injection cylinder at a low speed in the initial stage and at a high speed in the middle stage. The injection of the melt into the die casting machine is usually conducted by drive control of the injection cylinder, which is actuated by hydraulic pressure. To improve the quality of the molded products, low-speed injection must be switched to high-speed injection as quickly as possible and the injection speeds during the low-speed injection and the high-speed injection must be maintained as stably as possible.
In order to automatically adjust the flow rate to some degree, electromagnetic flow adjusting valves of the spool type have heretofore been employed. Electromagnetic flow adjusting valves control the flow rate in proportion to the input current and are equipped with differential transformers to move pilot spools in proportion to a current supplied to the differential transformers. Electromagnetic flow adjusting valves are further interlocked to main spools, so that the flow rate is maintained at a setpoint value. Electromagnetic flow adjusting valves are suitable for adjusting the flow rate to a desired value, but have no means to stop the main spool at a preset position. Therefore, upon receipt of an external force, the main spools move back and forth in the axial direction with the preset position as a center, whereby the flow rate undergoes variation and the injection speed of the die casting machine is not maintained constant.
In recent years, on the other hand, it has been demanded to arbitrarily set a pattern of injection speed to meet the metal mold in order to further improve the quality of the molded products. With the conventional flow adjusting valves of the spool type, however, the screw shaft is manually operated to set a flow rate. Therefore, it was impossible to arbitrarily set a pattern of injection speed. For this purpose also, electromagnetic flow adjusting valves of the spool type have also heretofore been used. In the electromagnetic flow adjusting valves of this type, however, the fact that the valve spools are actuated utilizing magnetic force makes it very difficult to precisely and quickly switch a small flow rate of, for example, 10 to 20 liters a minute to a large flow rate of, for example, 50 to 100 liters a minute with a maximum of 15,000 liters a minute, with a single flow adjusting valve. Therefore, when electromagnetic flow adjusting valves of this type are adapted to a hydraulic circuit for driving the injection cylinder, separate valves must be provided for the small flow rate and for the large flow rate along with a switch valve.
Therefore, conventional electromagnetic flow adjusting valves of this type have not been able to satisfy such requirements as quick switching of injection speed, stable injection speed during the moment of switching, and simplicity of the hydraulic circuit. Accordingly, they have not been well suited for highspeed injection application, which requires stable injection speed and speed changes after short periods of time.
Further, even small amounts of foreign matter which are contained in the operating oil and which adhere on the pilot spool cause its movement to be changed and the flow rate to be changed.
Moreover, when the hydraulic circuit is switched, there develops shock in the hydraulic pressure due to the change of pilot lines and change of valves, to disturb operation of the main spool which has no means for mechanically anchoring it at the present position. Accordingly, it becomes difficult to accurately adjust the flow rate. This adversely affects the injection and the quality of the injection-molded products.
According to the conventional art, furthermore, a total of four valves were necessary; i.e., a switch valve for low-speed injection, a flow adjusting valve, a switch valve for high-speed injection, and a flow adjusting valve. Therefore, the hydraulic circuit tended to become complicated, requiring cumbersome control operation.
In connection with the above, it is noted that a flow adjusting valve of the spool type has heretofore been used in which one end of a cylinder chamber or a valve bore formed in the valve body serves as an inlet port for introducing the fluid that is to be controlled, and the opening degree of an outlet path formed in a side portion of the cylinder chamber is adjusted by moving the valve spool which is slidably provided in the cylinder chamber in the axial direction. However, in the conventional flow adjusting valve of this type in which the valve spool slides in direct contact with the inner surface of the valve body, the cylinder chamber must be formed in the valve body requiring highly precise machining, which is difficult to accomplish. Further, since it is difficult to absorb misalignment of axis between the cylinder chamber of the valve body and the valve spool, excessive force is exerted on the valve spool or on the driving portions, making it difficult to smoothly move the valve spool.
Under the above prior art circumstances, the present inventor made the invention disclosed in Japanese patent and U.M. applications, published unexamined under the Numbers: No. 58-102878 (U.M.), No. 59-89869 (Pat.) and No. 59-89870 (Pat.), and also is described in the corresponding U.S. patent application Ser. No. 455,512, now U.S. Pat. No. 4,586,539, and United Kingdom Patent Application of which publication No. is GB No. 2112908A.
According to that prior invention, there was provided a single flow adjusting valve of the spool type in place of the conventional plural valves incorporated in a single hydraulic conduit line in the die casting machine and eliminating the defects inherent in the conventional art.
The single flow adjusting valve has advantages in the following matters.
First, the valve is capable of precisely, quickly, and automatically adjusting the flow rate and in which the single valve works as both a flow adjusting valve and a switch valve for low-speed and high-speed injection. Second, it enhances the performance of switching of the flow rate at high speeds by reducing the required driving force. Third, it prevents the accuracy for adjusting the flow rate from being decreased by external force or by change in temperature.
The above mentioned spool-type flow adjusting valve comprises: a valve body having a flow inlet and a flow outlet for transferring a pressurized fluid within the valve; a valve bore formed within the valve body to communicate with the flow inlet and the flow outlet and having first and second chambers at opposite ends of the valve bore, the flow inlet being connected to the first chamber; a valve spool slidably positioned within the valve bore to divide the valve bore into the first and second chambers and maintaining frictional engagement with the valve bore, the valve spool having a cylindrical rod for slidably positioning the valve spool along the axis of the rod between the first and second chambers, the surface area of the valve spool facing the first chamber being greater than the surface area of the valve spool facing the second chamber, the opening and closing of the flow outlet being dependent upon the reciprocating movement of the valve spool within the valve bore; means for slidably positioning the cylindrical rod; and at least one flow passage formed within the valve spool for interconnecting the first and second chambers, the flow passage being defined by a through-hole formed along the length of the valve spool. The flow of the fluid passing between the flow inlet and the flow outlet increases as the valve spool moves in the direction of the second chamber and opens the flow outlet.
The spool-type flow adjusting valve further includes a groove between the outer surface of the valve spool adjacent to the valve bore to open to the flow passage, for allowing the fluid to flow from the flow passage into the flow outlet when the groove is aligned with the flow outlet. The flow outlet includes first and second channels extending substantially perpendicular to the axis of the valve spool. The first channel is in fluid communication with the first chamber, and the second channel is in fluid communication with the flow passage when the groove is aligned with the second channel.
The means for slidably positioning the cylindrical rod includes: a housing mounted adjacent to the second chamber, the adjacent surfaces of the second chamber and the housing having receiving means coaxially aligned with the cylindrical rod; a connecting shaft slidably supported within the shaft receiving means and having a rod portion and a cylindrical portion, the rod portion being connected to the valve spool; motor means including a pulse motor having a driving member and a driven member mounted within the housing for imparting rotational movement within the housing; conversion means connecting the cylindrical portion of the connecting shaft to the driven member for transforming rotational movement into reciprocating movement, the conversion means including a nut attached to the cylindrical portion, a screw drive shaft connected to the driven member for rotation within the nut, balls rotatably positioned between the nut and the screw drive shaft and means for preventing the nut from rotating relative to the housing.
The spool-type flow adjusting valve is used in a die casting machine comprising an injection cylinder for activating a plunger by pressurized oil, a pressurized oil source, a single hydraulic passage through which the oil is supplied from the oil source to the injection cylinder, so that the single passage includes the spool-type flow adjusting valve therein in such arrangement that upstream and downstream portions of the single passage communicate with the flow inlet and the flow outlet of the valve, respectively, whereby the injection operation is switched from low speed injection to high speed injection.
The above spool-type flow adjusting valve used in the die casting machine, can be designed so that a thrust acting on the valve spool due to the pressurized oil behaves in the following manner. The thrust is in a valve opening direction when the degree of valve opening is zero and it is abruptly decreased as the valve opening degree is increased from zero to a certain degree. It is then increased as the degree is further increased. According to one design, the thrust is kept in a valve opening direction at any valve opening degree. However, according to another design, a direction of the thrust is changed from the valve opening direction to the valve closing direction at a certain degree of the valve opening.
Such alternative thrust behavior is advantageous in promoting or accelerating the valve opening movement of the spool to the effect that the valve is switched to have the flow rate of the oil from a low speed injection region (i.e., a small degree of valve opening) such as 0.1 to 0.2 m/sec to a high speed injection region (i.e., a large degree of valve opening) such as 1 to 5 m/sec) during a short period of time, e.g., 0.05 sec. In this case, it is desired that an absolute value of the thrust (in a direction of valve opening or closing) be as low as possible. This is because, otherwise, in the case of an increased valve capacity such as 50 to 15000 l/min, an extra and large force loaded by the driving and transmission mechanism is required to maintain the spool at a position where a desired high flow rate or high speed injection is obtained. The thrust not in the valve opening direction but in the valve closing direction at a high speed injection region appears to be more preferable, because such thrust exerts a braking force when the spool is going to stop. However, in a general evaluation, such a thrust in the valve opening direction is not always advantageous more than that of a thrust in the valve opening direction at the high speed injection region in that a period of time from the starting at the low speed injection region to the stopping at the high speed injection region is decreased while requiring a reduced driving force to move the spool.
In connection with this, it has been noted that even in the above mentioned other design to produce a thrust of the spool in the valve closing direction at the high speed injection region, the direction of such thrust changes from the valve closing direction to the valve opening direction if the spool is forced to move further toward the valve opening direction.
As explained above, the spool-type flow adjusting valve is advantageous in exhibiting high-speed performance for switching the flow rate, while requiring reduced driving force.
However, the inventor recently has found that, in using such flow adjusting valves, there is the following problem.
When an abnormality occurs in operation, where the load torque becomes greater than the motor torque, the supply of power is interrupted due to power failure or the spool shaft happens to break for some reason, the flow rate of pressurized oil flowing into the injection cylinder abnormally increases with the result that the flow adjusting valve, the driving means, and/or the die casting machine is damaged. Such a result is dangerous to the operator involved.